The present invention relates to a wireless communication system using a white space, and more particularly to a technology for assigning a frequency to be used.
Progress of an information society in recent years is very remarkable, and as a communication method for many information and communications apparatus and services, wireless communication is also used frequently other than wire communication. Therefore, the demands for the radio frequencies whose use is limited are increasing continuously, and the depletion of frequencies that can be allocated is posing a big problem in every country in the world. In general, a country performs license management of the frequencies, and only those who are licensed to use can use the frequencies in the specific location and time period. But, in order to satisfy the demands for frequencies which will increase continuously in the future, a new method of using the frequencies not being obsessed with the incumbent use method is being demanded.
Therefore, as a new method of using the frequencies to solve the depletion problem of the frequencies, studies are being made in these years to provide a method using a frequency band (white space) which is not used spatially and timewise though it has already been allocated. For example, research and development are being performed on a cognitive wireless communication system or the like by which an unlicensed user (hereinafter referred to as “secondary user”) uses the radio wave of the white space flexibly while the influence of the incumbent system for a licensed user (hereinafter referred to as “primary user”) to the frequency use is avoided sufficiently (See Non Patent Literature 1).
For example, in a wide-area wireless communication (WRAN: Wireless Regional Area Network) system using the white space, that is standardized by IEEE 802.22, respective radio stations access the database on an IP network to obtain a transmittable frequency list based on the position information on a local station and maximum transmittable power. The transmittable frequency list is collectively managed while it is being updated as required by a spectrum manager (SM) within a base station (BS) to which a terminal (CPE: Customer Premises Equipment) such as a slave station, cellular phone or the like, which is installed from door to door, is connected. And, the BS decides as a use frequency the frequency usable for two-way communication between the BS and the CPE based on the transmittable frequency list.
The respective radio stations (such as BS and CPE, the same applies below) are provided with a spectrum sensing function. Upon detecting by the spectrum sensing that the decided use frequency is being used by the incumbent system (system for the primary user), the respective radio stations notify the information to the SM. Then, the SM excludes the above frequency from the transmittable frequency list. The wireless communication system using the white space performs dynamic spectrum access based on the information which is updated every moment as described above, avoids an influence to the frequency use by the primary user and also realizes communication of the secondary user.
IEEE 802.22 defines time division duplex (TDD) only as the duplex operation system. The BS and the CPE use the same frequency at the time of data transmission according to the TDD, but realizes two-way communication by performing communication with a different transmission timing. However, the duplex operation system according to the TDD only uses the same frequency as the two-way communication. Therefore, for the wireless communication system using the white space involving a case where a transmittable frequency is different for the respective radio stations or a case where transmittable maximum transmission power is different, it is hard to say that the TDD is an efficient communication method. For example, in a case where one radio station can transmit with a high power at a certain frequency but the other radio station can transmit with a low power only at the same frequency, communication quality in two ways becomes asymmetrical. Therefore, when such a frequency is allocated, the communication cannot be performed efficiently.
There is also a case where transmitting and receiving in opposite directions can be performed even if one radio station can perform data transmission at a certain frequency but the other radio station cannot receive at that frequency. In such a case, it becomes a unidirectional link by which transmitting and receiving can be performed one way only, but such a frequency cannot be used as a matter of course by the TDD of IEEE 802.22.
Examples of prior art documents of the present invention are as follows.
Non Patent Literature 1: Koji Fujii, “Cognitive radio: Core technology of using white space to eliminate the waste of wave use”, [online], RIC TELECOM, [Retrieved on Jun. 9, 2011], Internet
Non Patent Literature 2: “IEEE Std 802.22-2011 Part 22: Cognitive Wireless RAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Policies and Procedures for Operation in the TV Bands”, edited by Institute of Electrical and Electronics Engineers (IEEE) Computer Society, (U.S.A.), IEEE Standards Association, Jul. 27, 2011